The present invention relates to a cobalt alloy for spinner discs used in rotary glass fiberization processes, and, in particular, to a cobalt alloy which extends the service life of spinner discs used in rotary glass fiberization processes or exhibits some other desirable property related to either the fabrication or use of such spinner discs.
Elevated temperature resistant cobalt alloys are typically used as the alloys for spinner discs in rotary glass fiberization processes. The cobalt alloys are used for this application because of the superior performance of such cobalt alloys in this application when compared to iron or nickel based alloys. The superior performance of the cobalt alloys is due to their higher strength and creep resistance at the elevated temperatures used in rotary glass fiberization processes and their greater corrosion resistance at the elevated temperatures used in rotary glass fiberization processes.
Cobalt alloys consist of a strong and corrosion resistant cobalt-chromium matrix (Co--Cr matrix), which is further strengthened, with a dispersion of coarse, strong carbides. The carbides in the microstructure are second phase strengtheners, the bulk of which are produced during the casting of the spinner disc in the solidification portion of the process. Cr.sub.23 C.sub.6 represents the dominant carbide by volume. However, Mo.sub.x C.sub.y carbides also form. These molybdenum carbides tend to be thermally stable, i.e. these molybdenum carbides melt at higher temperatures than the chromium carbides, yet the presence of molybdenum carbides can be extremely detrimental to certain hot corrosion, sulfidation environments, because molybdenum carbide alters the corrosion product chemistry and accelerates corrosion. Thus, while these molybdenum carbides impart improved elevated temperature strength to the cobalt alloys, the molybdenum carbides can accelerate attack in the existing short circuit path for oxidation, sulfidation and other forms of elevated temperature corrosion. The corrosion short circuiting is heavily impacted by near continuous morphology of the carbides. In addition, carbides have another detrimental characteristic, carbides are the last portion of the cobalt alloy microstructure to freeze on cooling and the first portion of the cobalt alloy microstructure to melt on heating. Hence, the phase in the cobalt alloy, relied upon for strength in the alloy, melts at the lowest temperature of the components forming the cobalt alloy.
The aforementioned characteristics of cobalt alloys, the structure-property-performance-process characteristics exhibited by different cobalt alloys, makes the development of new cobalt alloys with good high temperature corrosion and oxidation resistance property balanced with good high temperature mechanical strength very difficult. For example, chromium is a primary component for increasing the high temperature corrosion resistance of cobalt-chromium alloys. However, increasing the chromium content of a cobalt alloy beyond an optimum percentage of the alloy by weight will increase the volume fraction of carbides. This results in an imbalance in the desired physical properties for the cobalt alloy as there will be an increase in high temperature alloy strength but a decrease in alloy ductility. In addition, "excessive" carbides will generally reduce overall alloy corrosion resistance as there will be a greater volume of the short circuit carbide paths for corrosion. While the Co--Cr matrix of a cobalt alloy, with more than the optimum percentage by weight of chromium, can have good corrosion resistance, the Co--Cr matrix will tend to form weak and brittle Co.sub.x Cr.sub.y topologically close packed (TCP) phases due to the increase in electron vacancies trending the alloy from metallic bonding toward electron compounds. Thus, to be effective for a particular application, such as a spinner disc for the rotary fiberization of molten glass, the structure-property-performance-process properties of a cobalt alloy must be balanced for an optimum performance under the corrosive environmental conditions and the mechanical stresses encountered within the process envelope.